DEFAULT

where name is an arbitrary object and alist is a 1- or
2-dimensional array. This function returns the contents of the
:default field of the header of alist. When aref1 or aref2 is used
to obtain a value for an index (or index pair) not bound in alist,
the default value is returned instead. Thus, the array alist may be
thought of as having been initialized with the default value.
default operates in virtually constant time if alist is the semantic
value of name. See arrays.